In the plastic injection molding art, the usual challenges facing a product designer are to design an article having requisite strength for the product application and uniform surface quality for satisfactory appearance, but to avoid excessive weight, material usage and cycle time. A design compromise must often be made between strength and plastic thickness. A relatively thicker plastic section in the article, such as a structural rib, will incur greater weight, material usage, cycle time and induce sink marks and other surface defects due to thermal gradients in the area of the thickened section.
It is known in the plastic molding art to use pressurized fluid in conjunction with the plastic molding of articles. The pressurized fluid is typically nitrogen gas which is introduced into the mold cavity at or near the completion of the plastic injection. The pressurized fluid serves several purposes. First, it allows the article so formed to have hollow interior portions which correspond to weight and material savings. Second, the pressurized fluid within the mold cavity applies outward pressure to force the plastic against the mold surfaces while the article solidifies. Third, the cycle time is reduced as the gas migrates through the most fluent inner volume of the plastic and replaces the plastic in these areas which would otherwise require an extended cooling cycle. Fourth, the gas pressure pushes the plastic against the mold surfaces, thereby obtaining the maximum coolant effect from the mold.
However, as the dimensions of the molded article increase, the gas must do more work to migrate through the volume of the mold cavity to assist in setting up the article within the cavity. If the pressure of the gas is too great as it enters the mold cavity, there is a risk that it may rupture or blow out the plastic within the mold cavity, i.e., the gas is not contained within the plastic. Thus, there have been practical limitations in the adaptation of gas injection in the plastic molding field.
It is important to ensure that the maximum gas flow length in a channel is obtained so that permeation, shadow marks or sinkage are minimized. It is known that as the gas enters the partially filled molding and urges the plastic forward or outward to fill the cavity, that the gas opening or openings created in the molding get progressively smaller the further we get from the gas entry port. This is illustrated in FIGS. 5 and 6. This can result in the gas channel not being completely filled with gas to the end of the channel. This can result in sinkage at the end of the channel.
One way to overcome this problem is to reduce the amount of plastic injected into the cavity. This leaves more room for gas. However, this could lead to another problem which is the permeation of the gas into the walls of the molding. Further reduction in weight could cause blowout of the gas and a nonfilled molding.